This invention relates to integrated circuits such as programmable logic device integrated circuits, and more particularly, to circuitry for detecting and locating soft errors in integrated circuits.
Programmable logic devices are a type of integrated circuit that can be programmed by a user to implement a desired custom logic function. In a typical scenario, a logic designer uses computer-aided design tools to design a custom logic circuit. When the design process is complete, the tools generate configuration data. The configuration data is loaded into programmable logic device memory elements to configure the devices to perform the functions of the custom logic circuit.
Programmable logic device memory elements are often based on random-access-memory (RAM) cells. Because the RAM cells are loaded with configuration data during device programming, the RAM cells are sometimes referred to as configuration memory or configuration random-access-memory cells (CRAM).
During normal operation of a programmable logic device, loaded CRAM cells produce static output signals that are applied to the gates of metal-oxide-semiconductor (MOS) field-effect transistors (e.g., pass transistors). The CRAM output signals turn some transistors on and turn other transistors off. This selective activation of certain transistors on the device customizes the operation of the device so that the device performs its intended function.
When operating in a system, programmable logic devices are subject to environmental background radiation. Particularly in modern programmable logic devices that contain large numbers of CRAM cells, there is a possibility that a radiation strike on a CRAM cell will cause the CRAM cell to change its state. For example, a CRAM cell storing a “one” configuration bit may flip its state so that the cell erroneously stores a “zero” bit. When a CRAM cell changes state, the transistor being controlled by that CRAM cell will be placed into an erroneous state. The transistor might, for example, be turned on when it should have been turned off.
Radiation-induced errors that arise in configuration random-access-memory cells are sometimes referred to as soft errors. One way in which soft errors can be addressed is to change the design of the configuration random-access-memory cells. However, changes to the configuration random-access-memory cells can introduce undesired circuit complexity and can consume additional circuit real estate. As a result, it is often necessary to use a configuration random-access-memory cell architecture that occasionally produces soft errors.
Conventional programmable logic devices include error detection circuitry that continuously monitors an entire array of configuration random-access-memory cells. If an error is detected in the array, an error flag may be set. Systems that require highly reliable operations can monitor the status of the error flag to determine whether the programmable logic device has experienced any soft errors. So long as no errors are present, the system allows the programmable logic device to operate normally. If, however, the state of the error flag indicates that one of the configuration random-access-memory cells on the device has exhibited a soft error, the system can reconfigure the device by reloading the original configuration data into the configuration random-access-memory cells.
Because most soft errors do not produce permanent circuit damage, the process of reloading the original configuration data into a programmable logic device that has experienced a soft error is generally successful in restoring normal system operation. However, the reloading process can be disruptive to system operations, particularly when the programmable logic device is being used in a sensitive system in which fault-free operation is desired.
In some situations, soft errors in CRAM cells do not disrupt device operation. For example, a soft error in a CRAM cell that is not being used to implement part of a user's logic design may not adversely affect the normal operation of a device. With conventional programmable logic devices it is generally not possible to determine whether or not critical circuitry will be affected by a soft error, because conventional error detection circuitry is unable to determine the location of the soft error.
It would therefore be desirable to be able to provide integrated circuits such as programmable logic device integrated circuits with circuitry for detecting and locating soft errors in configuration random-access-memory.